1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring in a non-destructive manner the thickness of the overlay clad of the pressure vessel of a nuclear reactor.
2. Prior Art
An overlay clad of stainless steel is provided on the inside of a vessel made of a low alloy steel in a pressure vessel of a nuclear reactor and it is necessary to measure in a non-destructive manner the thickness of the overlay clad in the course of the testing of the soundness of this pressure vessel of the nuclear reactor. According to a prior-art, in ultrasonic wave measurement method has been proposed which method utilizes the reflection of the ultrasonic wave from the interface between the overlay clad and the low alloy steel.
According to the ultrasonic wave measurement method of a prior art, since the densities of and the sonic velocities in the stainless steel and the low alloy steel are respectively almost the same as each other, the reflection of the ultrasonic wave from the interface of the stainless steel and the low alloy steel where they are well integrated by means of weld penetration is not sufficient to allow accurate measurement. In fact, the thickness of the overlay clad of the stainless steel applied to the inner surface of the pressure vessel of a nuclear reactor is in the order of 5-10 mm and measurement of the thickness of that overlay clad is preferably executed with a resolution in the order of 0.1 mm for practical usage. It is to be pointed out, however, that the ultrasonic wave measurement method according to a prior art is unable to attain such a desired accuracy.